CD47 (also known as integrin associated protein [IAP]) is a transmembrane protein that belongs to the immunoglobulin superfamily and binds to several known partners, including: membrane integrins, thrombospondin-1 (TSP-1) and signal-regulatory protein alpha (SIRPα). CD47 is associated with a range of cellular processes, including apoptosis, proliferation, adhesion, and migration of cells and, importantly, it plays a key role in immune and angiogenic responses. CD47-SIRPα signalling is a critical molecular interaction that inhibits the activation of phagocytosis by macrophages and other myeloid cells. This promotes the survival of tumour cells and therefore acts as a myeloid lineage-specific immune checkpoint.
Preclinical evidence suggests that blocking CD47-SIRPα signalling can enhance the phagocytic activity of macrophages and inhibit the growth of xenografts in numerous experimental models of both haematological and solid malignancies. As macrophage activity is also a recognised factor in the biology of inflammation-associated tissue remodelling such as tissue fibrosis and the formation of atherosclerotic plaques, the CD47-SIRPα signalling axis is also of considerable therapeutic potential in non-cancerous diseases. Hence, anti-CD47 mAbs have the potential to act as immunotherapeutic agents in cancer and other settings, and to amplify the effectiveness of currently established therapies.
The majority of currently approved antibody therapeutics are derived from immunized rodents. Many of those antibodies have undergone a process known as “humanization”, via the “grafting” of murine CDRs into human v-gene framework sequences (see Nelson et al., 2010, Nat Rev Drug Discov 9: 767-774). This process is often inaccurate and leads to a reduction in target binding affinity of the resulting antibody. To return the binding affinity of the original antibody, murine residues are usually introduced at key positions in the variable domain frameworks of the grafted v-domains (also known as “back-mutations”).
While antibodies humanized via CDR grafting and back mutations have been shown to induce lower immune response rates in the clinic in comparison to those with fully murine v-domains, antibodies humanized using this basic grafting method still carry significant clinical development risks due to the potential physical instability and immunogenicity motifs still housed in the grafted CDR loops. Antibodies such as CD47 inhibitors that target receptors on immune cells, and whose pharmacological function is to stimulate immune responses via antigen presentation, are at heightened risk of provoking anti-drug antibody responses. These anti-drug antibody responses in the patient can reduce drug half-life, potency and safety during clinical use. As animal testing of protein immunogenicity is often non-predictive of immune responses in man, antibody engineering for therapeutic use focuses on minimizing predicted human T-cell epitope content, non-human germline amino acid content and aggregation potential in the purified protein.
The ideal humanized antagonistic anti-CD47 antibody would therefore have as many identical residues as possible in the v-domains to those found in both the frameworks and CDRs of well-characterized human germline sequences. Townsend et al. (2015; PNAS 112: 15354-15359) describe a method for generating antibodies in which CDRs derived from rat, rabbit and mouse antibodies were grafted into preferred human frameworks and then subject to a human germ-lining approach termed “Augmented Binary Substitution”. Although the approach demonstrated a fundamental plasticity in the original antibody paratopes, in the absence of highly accurate antibody-antigen co-crystal structural data, it is still not possible to reliably predict which individual residues in the CDR loops of any given antibody can be converted to human germline, and in what combination. CDR germ-lining is thus a complex, multifactorial problem, as multiple functional properties of the molecule should preferably be maintained, including in this instance: target binding specificity, affinity to CD47 from both human and animal test species (e.g. cynomolgus monkey, also known as the crab-eating macaque, i.e. Macaca fascicularis), v-domain biophysical stability and/or IgG expression yield. Antibody engineering studies have shown that mutation of even single residue positions in key CDRs can have dramatic effects on all of these desired molecular properties.
WO2014/093678A2 describes an antagonistic murine anti-CD47 IgG molecule termed “VxP037”, and also the preparation of humanized forms of VxP037. Those humanized forms of VxP037 were produced using classical humanization techniques, i.e. by grafting of Kabat-defined murine CDRs into human heavy and light chain framework sequences, with some of the human framework residues being potentially back-mutated to the correspondingly positioned VxP037 murine residues. For reasons noted above, such humanized forms of VxP037 described in WO2014/093678A2 are not ideal.
The present invention provides a number of optimized anti-CD47 antibodies and medical uses thereof.